1. Field of the Invention
The present invention relates to a rechargeable electrical energy storage device, and, more particularly, to a compact modular rechargeable cell construction which utilizes a molten salt electrolyte.
2. Prior Art
The problem of air pollution in urban areas attributed to emissions from motor vehicles using internal combustion engines is of increasing concern. Because battery-powered vehicles themselves produce no exhaust or unburned fuel emission, they are particularly attractive for urban use. However, to develop practical electrical automobiles for general use, low-cost secondary batteries having sufficient high-energy density and high-power density are required. Liquid lithium metal has been extensively utilized in some high-power density molten salt batteries, e.g., Li/Cl.sub.2, Li/S, Li/Se, and Li/Te.
The lithium-sulfur cell using molten halide electrolytes is of particular interest. See M. L. Kyle et al., "Lithium/Sulfur Batteries for Electric Vehicle Propulsion", 1971 Sixth Intersociety Energy Conversion Engineering Conference Proceedings, p 38; L. A. Heredy et al., Proc. Intern, Electric Vehicle Symp., Electric Vehicle Council 1, 375 (1969). However, it has been found that high self-discharge rates due to corrosion of cell components by liquid lithium coupled with some appreciable solubility of liquid lithium in the molten salt electrolytes often cause difficulties in material selection and battery cell design. Such difficulties can be avoided by use of a solid alloy of lithium as a source of lithium in an electrochemical cell. One such alloy is the aluminum-lithium alloy which has been utilized as the solid negative electrode. Excellent electrochemical performance of aluminum-lithium alloy in a composition range of 5-30 wt.% lithium in a molten salt electrolyte has been reported. See N. P. Yao et al., "Emf Measurements of Electrochemically Prepared Lithium-Aluminum Alloy", J. Electrochem, Soc. 118, 1039-1042 (July 1971) and references cited therein.
Heretofore, the principal reported effort toward the development of a high-energy-density battery has been directed toward improvement of the individual active components of the cell or battery, namely, the electrodes and electrolytes. There exists, however, another problem in the development of such a battery, namely, a battery which utilizes a molten salt as the electrolyte is subject to sustained high-temperature operation as well as to considerable variation in temperature. Specifically, the battery temperature may range from ambient up to its operating temperature which normally is in excess of 200.degree.C, generally from about 350.degree. to 450.degree.C. The individual components of the battery have greatly different thermal coefficients of expansion. Obviously, some means must be provided in the battery for maintaining the individual components in a substantially fixed relationship with one another at these elevated temperatures and over the given temperature range.
It has been proposed that the components be fixed in a certain spatial relationship by utilizing a non-resilient means such as ceramic spacer members and pins to key the components together. However, such a technique does not allow for any movement of the components upon heating of the battery to its operating temperature. With a fixed restraining means, breakage of the individual components as a result of thermal expansion is not uncommon. Obviously, there still is a need for an electrical energy storage device wherein the individual components are retained in a desired relationship to one another, but which will still provide for some movement of the components as a result of thermal expansion.
Furthermore, in order to retain the active material in the positive electrode assembly, a porous separator is used. A relatively bulky supporting and retaining structure is then needed to hold the separator in place. To the extent that the available space in the cell structure is not utilized by active material, a lower energy density results. Thus in order to obtain a high-energy-density battery composed of a plurality of cells, which will be of practical interest for automotive applications and the like, the need exists for providing cell modules having maximum utilization of the cell volume, while at the same time providing ready access to cell components, ease of assembly, and reliability over sustained periods of operation. The devices heretofore proposed have been generally found lacking in several of these requirements.